Method and apparatus for protecting uultraclean surfaces

ABSTRACT

A method for protecting an ultraclean surface of an object includes the step of forming, from an ultraclean fluid, a laminar flow moving over the surface. In order to enhance the reliability of protection, the velocity in the layer of the laminar flow from the ultraclean is varied according to the height thereof in a cross-section perpendicular with respect to the direction of the propagation of the flow.

The invention relates to a method and apparatus for protecting a surfaceand, in particular, thought not exclusively, to a method and apparatusemploying a flow of a protective fluid. The invention may be used, forexample, in the manufacture of electronic equipment, in precisemechanical engineering and instrumentation, and medical engineering forthe treatment of abacterial chambers.

Modern industrial production is characterized by the use of materialshaving a high uniformity of composition and accuracy of machining (CleanRooms. Ed. By Hayakawa. 1990. Mir Publishing House, Moscow, pp. 44-58).It is impossible to manufacture such products without creating a specialproduction environment which is capable of ensuring predeterminedconditions in production rooms for long periods of time. It is mostefficient to maintain an ultraclean environment in production rooms withlaminar air flow (laminar flow is a flow in which the intensity oftransfer processes is close to a level determined by molecularmovement).

The velocity of contaminant particles in a laminar flow is determined bymolecular movement, and such velocity is several times lower than thevelocity of the flow proper. For this reason the particles can penetrateonly to a small depth into the flow because they are very soon washedoff the flow. In such a case, the degree of contamination of a surfaceto be protected depends on the degree of cleaning of the laminar flowproper. In addition, the direction of air flows carrying fine particlescan be predicted with the laminar flow so as to prevent them fromgetting to the surface being protected. For this reason this circulationmethod is most suitable and is widely used for producing high-purityfluids.

Vertical or horizontal circulation has been used so far for maintaininga high cleanliness of the environment within clean rooms.

Two methods for protecting surfaces are known. One method resides inwashing off, with a laminar air flow, fine particles released from asource of contamination to reduce the contamination down to such aslevel as to prevent the penetration of the particles into a zone near tothe surface being protected. The second method involves forming a flowof highly clean air in a zone near to the surface being protected inorder that it moves in parallel with the surface so as to prevent fineparticles from the interior space of the room getting to the surface ofan object being protected. This method of protection is efficient, butit has a number of disadvantages and, in particular, it calls forsubstantial investment, i.e. the special equipment of the rooms and ahigh operating cost. With this method, gases and vapors released atvarious production steps propagate throughout the entire room, and ahigh degree of cleaning of the incoming flow is necessary. This resultsin the need to have multiple cleaning filters, and the release of dustin the room is to be prevented so that requirements imposed uponclothing, equipment and operating procedures are rather stringent.

In the manufacture of miniature products which require especially cleanconditions (Clean Rooms. Ed. by Hayakawa. 1990. Mir Publishing House.Moscow. pp. 58, 59), e.g. in making extra ELSIs, the effectiveprotection of the surfaces of products against dust can be ensuredwithin a relatively small space with an ultraclean environment. Themethod involves producing a laminar flow of an ultraclean fluid to flowover the surface being protected.

This method allows power consumption to be reduced, since the size ofventilation and filtering devices is smaller; and the requirementsimposed upon the cleanliness of the whole room are lower. The systemdoes not call for protective chambers so that it can be integrated intoa continuous automatic production process. The method for forming a flowof an ultraclean fluid does not, however, allow turbulence to beavoided, hence it is not reliable enough.

The above-described method may be carried out by means of an apparatusfor providing a local clean space (Clean Rooms. Ed. by Hayakawa. 1990.Mir Publishing House. Moscow. pp. 58, 59). The apparatus has an aircleaning unit connected through a pipeline to a laminar flow formeraccommodated in a casing. The flow, which is thus formed, is directed toa zone in which an object being protected is located.

This apparatus has the same disadvantages as the above-described method.

The provision of such flow is also important in applications where it isdesired to protect the surface of an object not only againstcontaminants, but also against the air environment. In this case asteady laminar flow, which does not mix with the air environment andwhich is formed from a gas neutral with respect to the surface of anobject being protected, constitutes a reliable screen against thenegative effect of the air environment.

Depending on the condition of the air environment, i.e., on thevelocities of air flows, the variation of the velocity of the laminarflow can be effected by various means in arrangements to be described.If an external flow moves at a low velocity, the velocity of the laminarflow is monotonously or continuously decreased according to the heightof its layer in the direction away from the surface of an object beingprotected. A more stable laminar flow is thus produced. If the externalflow moves at a substantial velocity, it is preferred, in order toproduce a more stable laminar flow, to monotonously increase thevelocity of the laminar flow according to the height of its layer in thedirection away from the surface of an object being protected. Thislowers perturbations in the laminar flow since the velocities of theexternal flow and laminar flow are identical.

If the temperature of the surface of an object differs from thetemperature of the environment, or from the temperature of the laminarflow, the flow velocity is to be increased to prevent temperature flowsor to avoid perturbations that can arise in the laminar flow. However,if the velocity of a laminar flow increases, the flow becomes lessstable, and a perturbation on the part of an object being protected cancause turbulence. To avoid this, the velocity of the laminar flow ismonotonously increased from zero and then monotonously decreased to zeroaccording to the height of the layer of the laminar flow.

A method according to the invention can be carried out by means of anapparatus for producing a flow of a protective fluid, comprising ahollow casing having an inlet for admitting the fluid into the casingand a laminar flow former provided inside the casing, with a partitionwall pervious to the protective fluid being provided downstream of thelaminar flow former, the partition having a monotonously varyingresistance to the flow in at least one of orthogonal directions in aplane drawn perpendicularly with respect to the direction of the laminarflow.

Apparatus to be described below for producing a flow of a protectivefluid allows the above-described method to be carried out in the mostsimple way. The pervious partition may be designed in various ways inapparatus according to the invention. Thus, the partition may haveparallel passages of identical cross-sectional areas equally spaced overthe whole surface area of the partition, the lengths of the passagesincreasing in the plane of the longitudinal section of the partition atleast in one direction according to the height thereof. The monotonouslyor continuously increasing lengths of the passages offers increasingresistance to the protective fluid. The velocity of the laminar flowthat has passed through such a partition monotonously decreasesaccording to the height of the flow layer thus formed, and this gradualvariation of velocity of the laminar flow reduces the rate of growth ofperturbations in the flow, i.e., it rules out the appearance ofturbulence and, as mentioned above, reduces the thickness of the layercontaining particles from the environment, whereby reliability of theprotection is enhanced.

The pervious partition may be in the form of a plate having equallyspaced rows of holes. In one embodiment, the holes of one and the samediameter are arranged with a spacing of the holes monotonouslyincreasing in each successive step. In another embodiment the diameterof the holes monotonously increases in each successive row, the spacingremaining unchanged. it is easier to make this pervious partition, thana partition in which the lengths of the holes varies, and the thicknessof such a partition may be small, so that the apparatus is more compact.

A pervious partition, which is most easy to make, can be manufacturedfrom an open-porosity material which has an impervious screen on theside of the laminar flow former which partly covers the laminar flow,the thickness of the pervious partition being at least equal to theheight of its portion free from the impervious screen.

Embodiments of the invention will now be described, by way of example,with reference to specific non-limiting embodiments illustrated in theaccompanying drawings, in which:

FIGS. 1a-c is a cross sectional of view an apparatus having a perviouspartition in the form of a plate having holes;

FIG. 2 is a cross sectional view of an apparatus having a perviouspartition with passages of different lengths;

FIG. 3 is a cross sectional view of a first embodiment of an apparatushaving a pervious partition made of an open-porosity material

FIG. 4 is a cross sectional view of a second embodiment of an apparatushaving a pervious partition made of an open-porosity material.

Reference will first be made to FIG. 1, in order to enable there to be abetter understanding of a method for producing a flow of a protectivefluid using apparatus according to one aspect of the invention and thegist of methods according to the invention to be made clearer.

The apparatus shown in FIG. 1a has a hollow casing 1 having an inlet 2for admitting a protective fluid. The casing 1 accommodates a laminarflow former 3 in the form of a set of woven nets in a frame with a meshsize of 0.2 to 0.3 mm fastened inside casing 1 by means of spacers 4 and5. A pervious partition 6 having a support holder 7 is intimatelypressed against spacer 5. The configuration of the support holder 7conforms to the shape of the casing 1. The support holder 7 is insertedinto the casing 1 and has a shoulder around its external perimeter, theoutside dimensions of the shoulder correspond to the inside dimensionsof the casing 1, the thickness of the shoulder being sufficient forpervious partition 6 to be pressed against the spacer 5. One of thewalls of the support holder 7 has a recess for receiving a part to beprotected, e.g., a silicon wafer 8. The depth of the recess correspondsto the thickness of the wafer 8.

The pervious partition 6 (FIG. 1b) has several rows of holes 9 ofidentical diameter. The spacing of the holes 9 in each row increasesmonotonically from the lowest to the uppermost row. Accordingly, theresistance of the pervious partition 6 to the flow of the protectivefluid increases with the height of the partition.

The apparatus is used in the following manner.

Depending on a problem, either ultraclean air or ultraclean gas neutralwith respect to an object being protected is admitted through the inlet2 to the apparatus. The velocity of the flow admitted through the inlet2 is higher than the velocity at the outlet of the apparatus by the samefactor by which the cross-sectional area of casing is greater than thecross-sectional area of inlet 2. For instance, if the velocity at theinlet 2 is 30 m/s, a velocity of 1 m/s, which is suitable to maintainthe laminar character of the flow, is be obtained at the outlet of theapparatus with an appropriate ratio between the above-mentionedcross-sectional areas. When the flow passes through the former 3,perturbations in the flow are reduced, and the flow becomes uniform andlaminar.

Pervious partition 6 has a flow resistance to the laminar flowing fluidincident thereon that varies from its lower to its upper edge to reducethe velocity of the laminar flow according to the height thereof, i.e.,the maximum velocity of the protective fluid is reached at the surfaceof the wafer 8 being protected and the minimum value obtains at theupper boundary of the flow. Maximum velocity of the flow at the outletof the apparatus is as high as 1 m/s. In this embodiment it is veryimportant that the wafer 8 does not protrude out of the recess, to avoideventual perturbations in the flow that might otherwise causeturbulence.

If the ambient environment were to have a substantial velocity it wouldbe necessary to increase the velocity of the laminar flow in order torule out perturbations in the laminar flow. In this case the velocity isat its maximum in the upper layer with respect to the wafer 8 and thevelocity value is a minimum at the surface of wafer 8. With a velocityfront which is thus formed, the thickness of the wafer 8 may be greaterthan the depth of the recess of the support holder 7. A low velocity ofthe laminar flow at the surface of a protruding wafer 8 does not causeperturbations in the flow and will not disrupt its laminar character.The pervious partition 6 (FIG. 1b), with holes 9 of one and the samediameter, has spacing between the holes of a row which monotonicallydecreases row by row according to the height thereof. This arrangementof the pervious partition 6 allows the above-described velocity profileto be formed.

As mentioned above, with a temperature difference between theenvironment or laminar flow and the wafer 8, convection flows arise thatmight break the laminar flow. To avoid the effect of convection flows,the maximum velocity of the laminar flow is increased. To prevent theeventual turbulence that might occur at high flow velocity, the velocityvalue is increased from zero up to its maximum and then again decreaseddown to zero. For that purpose the spacing between the holes 9 of thepervious partition 6 is decreased in an embodiment which is notillustrated in each successive row beginning with the second one to acertain value and is then increased in further rows back to the initialvalue.

In FIG. 1c is illustrated a pervious partition 6 which can be used inthe apparatus of FIG. 1a to produce the same result as is obtained withthe pervious partition 6 of FIG. 1b. The difference resides in the factthat the variation of the resistance of the partition to a protectivefluid according to height is achieved by varying the diameters of theholes 9 in each successive row. Thus the holes 9 in the lower most roware of a larger diameter than the holes 9 in the uppermost row.

In another embodiment of pervious partition 6, (not shown) the spacingbetween and the diameters of the holes 9 are both varied in eachsuccessive row of holes.

It will be apparent from the above that a feature of a method accordingto the invention for protecting ultraclean surfaces of objects residesin producing a laminar flow of a protective fluid, with the velocity ofthe flow being monotonically varied within a layer of the flow accordingto the height thereof in a cross-section drawn perpendicularly withrespect to the direction of propagation of the flow. The flow shouldmove over the surface of an object being protected. This method offorming the laminar flow allows its properties to be maintained, e.g.over a length of flow of 1 m, with a thickness of the flow being formedof 0.04 m. Therefore, if an object is placed within such a spaceincorporating a protective fluid, reliable protection of a surface beingprotected against the penetration of contaminating particles from theenvironment, or against penetration of a gas towards the surface beingprotected from the environment can be achieved.

Other embodiments of a pervious partition are possible which allow asteady laminar flow to be formed over a limited length so as to ensurereliable protection of the surface of an object.

Parts of the apparatus shown in FIG. 1 and identical parts of theapparatus shown in the following figures are indicated by the samereference numerals.

FIG. 2 is a cross sectional view of an apparatus having a perviouspartition 6 which has passages 10 of one and the same diameter. Thelengths of the passages 10 in each successive row, beginning with thelower row, increase monotonically so as to offer an increasingresistance to the protective flow. This partition 6 can also be made bysuitably supporting capillary tubes arranged in a wall.

Most noteworthy are embodiments of the apparatus shown in FIGS. 3 and 4.In these embodiments a pervious partition 6 is made of an open-porositymaterial and is partly covered by a screen which is impervious to theprotective fluid. The thickness d of the pervious partition 6 should inthese embodiments, be at least equal to the height h thereof which isfree from the screen, i.e., d≧h. It is only this ratio between thethickness and height of the free portion of the pervious partition thatallows the flow to move in both longitudinal (in the plane of thedrawing) and transverse directions. The flow moving in the transversedirection encounters a greater resistance and its velocity decreases.Therefore, a smooth velocity profile of the laminar flow is formed inthis apparatus.

The difference between the apparatus shown in FIG. 3 and 4 is in theconstruction of the impervious screen. In the apparatus of FIG. 3 theimpervious screen is in the form of a wall 11 of the hollow casing 1which functions as a screen. In the apparatus of FIG. 4 a liner 12 fillsa part of the interior of the hollow casing 1 between the former and thepervious partition 6.

The pervious partition 6 can be made of sintered balls, felt andcellular materials. Other structures for the pervious partition 6includes a set of arrays, the spacing of the arrays being at least equalto the mesh size of the array. Such a construction of the previouspartition 6 is of special interest since the resistance of the perviouspartition 6 can be varied in both the longitudinal and transversesections by increasing the spacing of the arrays so as to vary thethickness of the flow formed in the apparatus. With an increase in thespacing of the arrays the resistance in the transverse section deceasesand the flow in this direction can reach the edge of the partition,i.e., maximum protective layer thickness can be obtained in this case.

Preferred embodiments of the invention have been described above, andother embodiments and modifications can be used without going beyond thescope of the invention as defined in the appended claims.

I claim:
 1. A method of protecting an ultraclean surface of an objectsituated in a contaminated surrounding medium, comprising the steps offorming, from an ultraclean fluid, a laminar flow layer of a pure fluidprotective medium that moves over the surface, progressively varying thevelocity of the laminar flow layer in at least one progression betweenan interface of the surrounding with the protective media and thesurface of the object in a cross-section perpendicular with respect tothe direction of propagation of the flow of the fluid, by(a) placing theobject in a recess of a base so a surface of the recess is generallyparallel to the protected surface of the object; (b) forming the laminarflow so it has a respective uniform velocity in each plane of thelaminar flow; and (c) causing the velocity of the laminar flowcontiguous to the base and the protected surface to be such thatperturbations of the laminar flow are minimized about any protrusionthat may be present on the surface of the object in the recess.
 2. Amethod as claim in claim 1, wherein the velocity of the laminar flow isdecreased through the layer in the direction away from the surface beingprotected.
 3. A method as claimed in claim 1, wherein the velocity ofthe laminar flow is increased through the layer in the direction awayfrom the surface being protected.
 4. A method as claimed in claim 1,wherein the laminar flow is formed so the velocity thereof increasesprogressively from zero and then decreases progressively to zero throughthe layer of the laminar flow.
 5. An apparatus for producing a laminarflow of a protective fluid for protecting a surface of an object,comprising a hollow casing having an inlet for admitting a pure fluidmedium thereto, a laminar flow former located in the casing downstreamof the inlet, a partition pervious to the protective fluid located inthe casing downstream of the laminar flow former, the partition having aresistance to the flow which varies progressively in at least one ofplural orthogonal directions of a plane perpendicular with respect tothe direction of movement of the laminar flow; the casing further havinga base with a recess in a surface of the base for accommodating theobject so the surfaces of the base and the object are parallel; thecasing including a structure downstream of the partition for causing thevelocity of the laminar flow to be uniform in every plane of flow in thehousing parallel to the surfaces and in a plane contiguous with theprotected object surface, consistently with any protrusion of the objectfrom the recess, for minimal disturbance to the laminar flow to enhancethe reliability of protection of the protected object surface.
 6. Anapparatus as claimed in claim 5, wherein the pervious partition has aplurality of parallel passages of one and the same cross-sectional areawhich are equally spaced over the entire surface area, the lengths ofthe passages increasing monotonically in the plane drawn perpendicularlywith respect to the direction of movement of the laminar flow.
 7. Anapparatus as claimed in claim 5, wherein the pervious partition is inthe form of a plate having equally spaced rows of holes of identicaldiameter, the spacing between the holes increases monotonically row byrow.
 8. An apparatus as claimed in claim 5, wherein the perviouspartition is in the form of a plate having equally spaced rows of holes,the diameters of the holes increasing monotonically row by row.
 9. Anapparatus as claimed in claim 5, wherein the pervious partition is madeof an open-porosity material and an impervious screen partly covers thepartition on one side of the laminar flow former, a thickness of thepervious partition being at least equal to the height of the portion notcovered by the impervious screen.
 10. An apparatus as claimed in claim 9wherein the pervious partition is in a form of a set of arrays, thespacing between the arrays being greater than the mesh size of thearrays.
 11. Apparatus for protecting an ultraclean planar surface of anobject comprising a hollow housing having (1) a planar floor, (2) arecess in the planar floor for holding the object so the surface to beprotected is exposed, the recess having a planar floor for receiving theobject and a height so the planar surface to be protected is flush withthe planar floor of the housing, and (3) means for supplying only alaminar flow of fluid to the surface and to a region beginning at thesurface and extending away from the surface at right angles to thesurface so the laminar flow has a progressively changing velocityprofile in the region as the distance from the surface increases. 12.The apparatus of claim 11 wherein the means for supplying the laminarflow having the progressively changing velocity profile includes abaffle having many apertures extending in the direction of the laminarflow, the baffle being positioned so it extends generally in a directionat right angles to the surface and has a thickness in planes generallyparallel to the surface, the fluid flowing through the apertures in thedirection of the baffle thickness.
 13. The apparatus of claim 12 whereinthe apertures are arranged so the flow resistance thereof in planesparallel and closest to the surface differs from the flow resistancethereof in planes parallel and remote from the plane of the surface. 14.The apparatus of claim 13 wherein the apertures are arranged so thecombined areas thereof varies as a progressively changing function ofdistance extending at right angles away from the surface so the combinedareas of the apertures in planes extending through the thickness of thebaffle close to the plane of the surface differ from the combined areasof the apertures in planes extending through the thickness of the baffleremote from the plane of the surface.
 15. The apparatus of claim 14wherein the combined area of the apertures close to the plane of thesurface exceeds the combined area of the apertures remote from the planeof the surface.
 16. The apparatus of claim 15 wherein the baffleincludes a plate having apertures in rows parallel to the plane of thesurface.
 17. The apparatus of claim 16 wherein the apertures in rowsclose to the surface have diameters greater than the apertures in rowsremote from the plane of the surface.
 18. The apparatus of claim 17wherein there are more apertures in rows close to the plane of thesurface than in the rows remote from the plane of the surface.
 19. Theapparatus of claim 13 wherein the apertures are in the form of elongatedpassages extending in the direction of the fluid flow such that thepassages in planes parallel and close to the plane of the surface havelengths different from the passages in planes parallel and remote fromthe plane of the surface.
 20. The apparatus of claim 19 wherein thepassages in planes close to the plane of the surface are shorter thanpassages in planes remote from the plane of the surface.
 21. Theapparatus of claim 13 wherein the baffle is formed of an open porositymaterial pervious to the fluid and having fluid flow paths in directionsparallel and transverse to the plane of the surface, the baffle beingarranged and flow paths of the baffle parallel to the surface being inthe direction of the laminar flow, the baffle being arranged and theflow paths transverse to the plane of the surface being such that thefluid incident on the baffle (a) initially flows into the baffle inplanes parallel to the plane of the surface having a first predeterminedextent in a direction at right angles to the plane of the surface, (b)thence flows through the flow paths in the baffle to exit from thebaffle in planes parallel to the surface having a second predeterminedextent in a direction at right angles to the surface, where the secondpredetermined extent exceeds the first predetermined extent.
 22. Theapparatus of claim 21 wherein planes parallel to the plane of thesurface in the second predetermined extent are closer to all planesparallel to the plane of the surface in the first predetermined extent.23. The apparatus of claim 21 wherein the first predetermined extent isnot greater than the thickness of the pervious material.
 24. Theapparatus of claim 21 wherein the baffle includes plural spaced screensin mutually parallel planes at right angles to the plane of the surface,the spacings between adjacent pairs of the screens exceeding the screenmesh size.